Present on a send response only when the alert reached NOBODY it was addressed to and the umbrella guarantee fired instead: the company owner was emailed that delivery failed.
This is the one case where success: true does not mean somebody was paged. The response reads as a success because the alert was rescued rather than lost, and because retrying it would only mail the owner a second time — but the person it was addressed to received nothing. Never report the alert as delivered when this object is present; report that delivery failed and the owner was told.
reason says what to repair, and it is the same vocabulary the rest of the API uses for delivery verdicts: no_deliverable_route (no channel could carry it — GET /recipients/{recipient} and GET /capabilities say which of the recipient, their contacts or the installation is the blocker), delivery_failed (routes existed and every attempt failed at send time), escalation_exhausted (the chain ran through every step and reached nobody), group_unreachable (a group fan-out reached none of its members).
What the email itself contains is an operator setting, not an API one: either the alert text, or — with the redacted mode switched on — only the fact, the reason and a link to the alert in the dashboard. Either way it is a real outbound email: it counts toward the daily anti-abuse ceiling, and it is reported separately from alerts in the usage report, because it is the consequence of a failed delivery rather than traffic the account asked for.